No wonder. Colonoscopy involves inserting a tube equipped with a lighted camera into your rear end and snaking it through the length of the colon looking for fleshy masses called polyps that may be cancerous or become cancerous.

Before the test, patients must take laxatives to ensure their bowel is empty. As clinicians sometimes say, between the bowel preparation and the test itself, the "ick" factor is pretty high.

Unfortunately, unless the test is performed by a skilled physician with a good record of detecting polyps, all that effort may be wasted. "The biggest problem with colonoscopy is that it's operator dependent," says Dr, Douglas Rex, director of endoscopy at Indiana University Hospital.

A skilled physician should have a polyp detection rate of about 25 percent for men and 20 percent for women, says Dr. Durado Brooks, director of prostate and colorectal cancers for the American Cancer Society. Those figures correspond to the percentages of men and women over 50 who have polyps.

Another key indicator is the "colonoscopy withdrawal time," which measures how long a physician takes to remove the scope after reaching the beginning of the colon. Longer is better, within reason. A withdrawal time of at least six minutes is associated with higher detection rates. Ten minutes is optimal, according to an analysis published last year.

"The colon is a very twisted organ, and you may see things on the way out that were missed on the way in," says Brooks.

Don't be shy. To make sure you're getting the most out of your once-a-decade screening test, quiz your gastroenterologist about those two measures, says Brooks.